In aerospace gas turbine engines, it is a common practice to have a gas turbine engine pneumatic system using high pressure air to control the open/closed condition of air bleed valves which are conventionally used, for example, in maintaining the operability of gas turbine engines. An example of a gas turbine engine pneumatic system for controlling the open/closed condition of an air bleed valve is schematically shown in FIG. 4 in which a pressure Px is applied to one side of a poppet valve against the high pressure P0 of compressed air acting on the other side of the poppet valve. The Px pressure is generated for example, by a simple dropping orifice network flowing air between a high pressure air source and a low pressure sink which is usually the ambient air pressure. The arrangement, in conjunction with the reference spring (not numbered), provides for a predetermined relationship between the valve open/closed positions and the pressure Px which in turn is based on the geometric features. For some gas turbine engine installations, it is necessary to adjust this relationship to provide additional engine operational flexibility, particularly during transient maneuvers. Accordingly, there is a need to provide an improved gas turbine engine pneumatic system for controlling an air bleed valve.